1. Field of the Invention
The present invention concerns a method to obtain a manganese steel alloy, also known as “Hadfield steel”, amagnetic, with an austenite structure, extremely tough and able to harden if subjected to repeated impacts and/or knocks. The steel alloy is normally used in applications where a high resistance to abrasive wear is required, such as for example in the industry to extract, process, grind, crush, riddle or suchlike, inert materials, stone, iron alloys or other.
2. Description of Related Art
A manganese steel alloy is known, whose properties were studied as far back as 1882 by R. A. Hadfield, who understood that a steel alloy with a percentage weight of manganese comprised between about 7% and about 20% is able to harden on the surface, that is, to create an extremely hard surface film which gives the alloy an optimum resistance to abrasion.
This manganese steel alloy, also known as Hadfield steel, represents the basic quality from which subsequently all the modifications of chemical composition were made, in order to make the use of this type of alloy more versatile. In fact, the prerogative of its resistance to abrasion is the development of the hardened layer which is generated through repeated impacts, that is, by means of cold plastic deformation.
One of the most important modifications to conventional manganese steel alloy is the introduction of chromium in different percentages of weight, which allows to obtain a harder austenitic matrix, and hence to use this type of alloy also in applications where the entity of the impacts are not such as to allow an optimum surface hardening of the alloy. By increasing the percentage of chromium it is possible to obtain on board and inside the austenite grain, after particular heat treatments, controlled an isolated precipitations of mixed carbides of chromium of a rounded form, which represent hard points such as to impede the mechanism of abrasive wear.
It must be observed that if the manganese steel alloy is subjected to slow cooling, which is the typical case of cooling that occurs in the flask after casting, it has a not completely austenitic structure, but one characterized by the presence of precipitates of pearlite and mixed carbides that continuously follow the edge of the austenitic grain. The presence of these islands on board the austenitic grain makes the material fragile and hence does not allow use of the steel alloy as it is cast.
A solution heat treatment is therefore necessary, that is, to heat the steel to a temperature comprised between 1000-1200° C. and subsequently to quench it drastically in water.
This treatment allows a solution heat treatment of the carbides and of the precipitated pearlite, giving a great toughness to the material thus treated.
In the case of manganese steel alloys with parts of chromium, however, the precipitation of carbides mixed with chromium on board the austenitic grain makes it necessary to perform solution heat treatments with controlled temperatures and according to heating duration times that are difficult to determine.
In fact, a competition is created between the time needed for the solution heat treatment of the carbides and the kinetics of the swelling of the austenitic grain, which latter phenomenon has to be contrasted so as not to negatively influence the alloys' properties of resistance to abrasion.
The problem of the solution heat treatment of the carbides is emphasized in the case of artifacts with thicknesses of more than 100 mm, since breakages may occur inside the material, during the quenching treatment in water, due to the presence of fragile zones which are not able to support the dilations due to the heat treatment.
In fact, the quenching means is not sufficient to allow rapid cooling also inside the section of the artifact, thus creating dangerous re-precipitations of carbides mixed with chromium which, in the subsequent cooling stages, make the austenitic structure excessively fragile.
It is also known that the strengthening of metal alloys is generally given by non-deformable particles present inside the crystal structure, that is, incoherent particles which do not allow themselves to be crossed by dislocations and which therefore increase the speed of hardening of a metal material, for example what happens in the process of ageing copper-aluminum alloys.
It is also known that an excess of aluminum and nitrogen in manganese steel alloys is deleterious for the structural solidity of the artifact, since the appearance of aluminum azides which are disposed on board the austenitic grain make the alloy fragile.
It is also known from the U.S. Pat. No. 4,531,974 to obtain manganese austenitic steel having a possible percentage weight of titanium comprised between 0.0% and 0.2% and a possible percentage weight of zirconium comprised between 0.0% and 0.05%.